Direct fabrication of arbitrary phase masks in optical glass via ultra-short pulsed laser writing of refractive index modifications

We study the possibility to fabricate an arbitrary phase mask in a one-step laser-writing process inside the volume of an optical glass substrate. We derive the phase mask from a Gerchberg–Saxton-type algorithm as an array and create each individual phase shift using a refractive index modification of variable axial length. We realize the variable axial length by superimposing refractive index modifications induced by an ultra-short pulsed laser at different focusing depth. Each single modification is created by applying 1000 pulses with 15 μ J pulse energy at 100 kHz to a fixed spot of 25 μ m diameter and the focus is then shifted axially in steps of 10 μ m. With several proof-of-principle examples, we show the feasibility of our method. In particular, we identify the induced refractive index change to about a value of Δn= 1.5 · 10 . We also determine our current limitations by calculating the overlap in the form of a scalar product and we discuss possible future improvements.